F

A star (or galaxy) which is in the area being viewed by an astronomer but is not the object that they are actually looking at.

Filter

Filters can be used to absorb certain colours or an amount of light to allow an object to be viewed more clearly. For example, filtering out a prominent colour while observing a planet makes it easier to view details of other colours. Observing the Sun through filters allows sunspots to be seen as a decreased amount of sunlight can pass through the filter.

Flare

A sudden burst of energy from a star, like the Sun.These usually occur on the Sun around sunspots. The energy is released from the Sun as flares which can extend many millions of miles from the Sun, emitting radiation and particles. If they reach Earth, they can cause radio interference. Solar flares can often be seen during eclipses or if the Sun is being observed through a telescope with a disc placed on the lens to block out the Sun's disc.

Flat Earth Society

Society originally based in England but now in California which believes that Earth is flat, despite overwhelming scientific evidence proving otherwise.

Flyby

Mission involving a spacecraft flying past an object (planet, moon, comet, asteroid) to take pictures and make observations of it. They were fairly common in the early years of space exploration before scientists were able to get crafts to land on objects or place them into orbits. They are also used if a spacecraft passes by an object of interest en route to another destination or is visiting multiple destinations as part of its mission (for example, Voyager 2 flew by the four Gas Giants and sent back images of all of them and some of their moons).